Before the Olympian Gods, there was a race of immortals called titans. These titans were the children of Gaea, the earth, and Ouranos, the sky. Among these children was Kronos. Kronos decided to overthrow his father, and cut him to pieces him with Ouranos's own weapon. He then threw the body into the ocean, and from the sea foam made of Ouranos's immortal essence, Aphrodite was born. After deposing his father, Kronos decided to rule with his wife and sister, Rhea. Together they had six children, but Kronos was afraid his children would overthrow him as he had his own father, and so each time Rhea gave birth, he would eat the child. Eventually, Rhea took pity on her youngest son, and swaddled a rock and gave it to her husband in place of their child. Kronos ate the rock, and Rhea hid Zeus away so he might grow up without being eaten by his father. Eventually, Zeus was grown, and decided to save his siblings by feeding his father a mixture of mustard and wine. This made Kronos throw up, and because they were immortal, Zeus's siblings were expelled alive, well, and fully grown. They all then worked together and killed Kronos the same way he had killed his own father: by cutting him to bits with his own weapon. They scattered the pieces of his body throughout Tartarus, the deepest part of the underworld, and exiled the rest of the titans to there as well. Thus the first gods were born: Aphrodite first, and the siblings soon after: Hestia, Demeter, Hera, Hades, Poseidon, and Zeus.
See also Who were the Gods